The present invention relates to a scanning device for an electronic photography type printer, and more particularly to an LED beam scanning device for a low speed electronic photography type printer, which scans an LED beam of an LED array to a desired position of the photosensitive drum by virtue of relative rotation of a rotary cylinder provided with spiral slits to a fixed horizontal slit plate provided with longitudinal slits.
Conventionally, known scanning device of an electronic photography type printer can be classified into various types, laser scanner type, hologram scanner type, LED light emitting diode) array type, LSC (liquid crystal shutter) type and the like. Among these types of scanning devices, the laser scanner type of scanning device has been widely used due to its fast printing speed and good printing result.
FIG. 1 is a perspective view showing a representative embodiment of a known laser scanner type of scanning device. As illustrated, the laser scanner type of scanning device comprises a laser diode 1 as a light source, a light source drive circuit 1' for driving the laser diode 1, a collimating lens 2 and a cylindrical lens 3, each for condensing the light beam having emitted from the laser diode 1, a polygon mirror 4 for reflecting the condensed beam, a drive motor and circuit assembly 5 for driving the polygon mirror 4 f-.theta. lens system 6, a photosensitive drum 11 disposed before and lower than the f-.theta. lens system 6, a totally reflecting mirror 7 disposeed at an inclination angle of 45.degree. between the f-.theta. lens system 6 and the photosensitive drum 11. Also, this scanning device is provided with a sensing beam reflecting mirror 8 disposed between the f-.theta. lens system 6 and the reflecting mirror 7, a beam sensing circuit 9 for sensing the reflected beam from the sensing beam reflecting mirror 8, an optical fiber 10 connected to the beam sensing circuit 9 for receiving and transferring the reflected beam. Here, the f-.theta. where lens system 6 satisfies the formulation .delta.Z=f..theta. (.delta.Z is the desired scanned position of the photosensitive drum 11, f is the effective focal distance, and .theta. is the scanning angle, that is the rotational angle of the polygon mirror 4) and comprises two lenses each having two bent surfaces with different respective radii of curvature.
In operation, the laser beam emitted from the laser diode 1 driven by virtue of the driving signal of the light source drive circuit 1' is condensed onto the polygon mirror 4 by way of the collimating lens 2 and the cylindrical lens 3. The polygon mirror 4 generally comprises a hexagon mirror body and rotates clockwise by the drive motor and circuit assembly 5. The laser beam having been condensed to the polygon mirror 4 is reflected to the f-.theta. lens system 6 wherein the effective focal distance f and the scanning angle .theta. of the laser beam are compensated, thereafter, the laser beam is scanned to the totally reflecting mirror 7. At this reflecting mirror 7, the laser beam is reflected at 90.degree. angle and then scanned to the photosensitive drum 11 with the effective focal distance and the constant scanning speed, thereby printing a desired image. Also, a part of the laser beam transmitted through the f-.theta. lens system 6 is reflected by the sensing beam reflecting mirror 8 so as to directed to the optical fiber 10, wherefrom the beam is transferred to the sensing circuit 9. The sensing circuit 9 determines the initial scanning position of the laser beam by analyzing said laser beam.
At this time, the scanning speed of the laser beam is constant, so that it is possible to scan the laser beam to a desired position of the photosensitive drum 11 by turning on the laser diode 1 after a predetermined time since the beam sensing circuit 9 determined the initial scanning position.
This type of scanning device has relatively good effect in printing speed and printing result. However, the scanning device has several disadvantages in that it includes several lenses, such as the collimating lens 2 and the cylindrical lens 3 each adapted for condensing the laser beam onto the polygon mirror 4, and the f-.theta. lens system 6 adapted for controlling the f-.theta. characteristics of the laser beam, thereby causing the scanning device to be complex and also it needs to provide an effective focal distance between the polygon mirror 4 and the photosensitive drum 11, so that compactness of the scanning device can not be accomplished. Furthermore, this type of scanning device has another disadvantage in that even minute error of each lens results in a serious error of the photosensitive drum 11 so that each said lens has to be machined in high accuracy, thereby causing the cost of the printer to be higher as a result of the high manufacturing cost of each lens.